A non-destructive method is provided for measuring a non-scattering coating on a non-specular or specular surface of a metallic substrate. The surface may be a rough surface, such as a chemically milled surface. Infrared energy is transmitted into an integrating sphere that is in physical contact with a sample of the coating on the metallic substrate. The infrared energy is partially absorbed by the coating. The infrared energy is in part specularly reflected by the metallic substrate and is in part scattered by the metallic substrate depending on the wavelength of the infrared radiation. The integrating sphere integrates and collects total reflectance of the infrared energy. Infrared detectors detect the total reflectance at two wavelength bands. A decrease in total reflectance in one of the two wavelength bands indicates presence of the coating or may be mapped to an amount of the coating.